1. Field of the Invention
The present invention relates to a display apparatus using a driving circuit substrate comprising a thin film transistor array and, in particular, to a transistor array suitable for the purpose that amorphous silicon is used as a semiconductor.
2. Description of the Prior Art
There is disclosed a display apparatus using a thin film transistor array as a driving switching element, for example, in U.S. Pat. Nos. 3,824,003 and 3,840,695, and "IEEE Transactions on Electron Device" Vol. Ed-20, No. 11 (November 1973), pp. 995-1001, etc. The display apparatus described in U.S. Pat. No. 3,824,003 is a display apparatus of the reflection type, in which liquid crystal is used as a display means. Drain electrodes serving as picture elements are made of metal of high reflectivity such as Al and the like to act as reflective plates.
The dynamic scattering effect has been conventionally employed to discriminate electro-optic change occurring in such apparatus. There have recently been used various liquid crystal operating modes exhibiting a field-effect. Examples of such relatively new modes are the twisted nematic mode (TN mode), the deformation of vertical aligned phases mode (DAP mode) using double refractionarity, the hybrid aligned mode (HAN mode), and the modified modes in which the above-mentioned modes are accompanied by the Guest-Host effect that the operation of the liquid crystal is readily discriminated in such a way that dichromic dyes are added in the liquid crystal. Such modes are disclosed in "Recent Display Apparatus" (Nippon Hoso Press Kyokai, 1974). In conventional apparatus, it is predicted that the above-mentioned modes may be optionally selected. However, when the present inventors investigated the display effect of such field-effect liquid crystals, they could not find an operating mode in which a sufficient indication effect can be obtained, in the conventional structures. It was found that the cause of the above-mentioned fact is based upon using electrodes with metallic mirror surfaces, which are also drain electrodes, as electrodes for display elements. The drawbacks of the mirror surface structure include action of the surface as a mirror in a transparent light view field upon view of the display, and therefore the optical change of the display is simultaneously seen with image of a background projected on the mirror surface. In other words, the background varies depending upon where the display apparatus is located, for example. The display surface is dark in the case of a dark background. However, the mirror surface structure results in formation of mirror images of light sources located on the ceiling or a face of an observer on the display surface, and therefore reading change of the display is sometimes difficult.
On the other hand, there is known a display apparatus using the above-mentioned dynamic scattering effect and having a display structure capable of being seen easily, in displays using conventional mirror surface metals. In this display apparatus, a liquid crystal cell is transparent when voltage is not applied, but the liquid crystal assumes the scattering state upon application of a voltage. A display apparatus is used in which a black background is located in the normally reflecting direction of the mirror surface of the display apparatus toward the viewing direction. In this display apparatus, rays of light incident on the display apparatus from surroundings, except for the normally reflecting direction, is scattered by the display portions (liquid crystal cell) when voltage is applied, so that an observer sees the scattering light. The observer sees the black background on transparent display portions when voltage is not applied, so that contrast between both portions is high. The primary drawback of the above-mentioned conventional structure is lack of reliability in comparison with the foregoing field effect mode, since the dynamic scattering effect consumes slight electric current. Second, the display tends to become dark, since light is not incident from the background portion on account of the black background. Further, the display using the dynamic scattering effect requires voltage higher than those of the foregoing other modes of the field effect type.
In conventional display apparatus, a coating alone has been used on a portion of a semiconductor layer where a liquid crystal material contacts for the purpose of enhancing stability of a thin film transistor, and preventing undersirable electochemical reaction between exposed portions of the liquid crystal material and the semiconductor layer. The materials which can be used for this purpose are calcium fluoride, a film of silicon dioxide, and quartz. However, in the case that amorphous silicon having excellent transistor characteristics is used, such protective coating is functionally insufficient to eliminate effects on the photoconductive characteristics, and stable operation of the liquid crystal can not be attained. In the case that the liquid crystal display is carried out by the field effect, it is not required that the surface of a drain electrode which is to be a display element unit contact with the liquid crystal. On the contrary, it is desirable that the surface of the drain electrode be covered with a transparent insulating film and for this reason the conventional structure is undesirable. Further, it is required that the liquid crystal molecules in a liquid crystal cell of the field effect type, be uniformly oriented. In other words, a treatment is required for obtaining a uniform surface of the liquid crystal. In view of this treatment, the conventional display structure has not been satisfactory.
For effecting display by using the conventional display apparatus, an electric field is generated between a selected drain electrode (display electrode) and a counter electrode in such a way that an image signal is applied to a gate line with scanning, and with a driving voltage to a source line. In this case, the display is carried out in such a way that an electro-optical change of the liquid crystal is read by detecting means such as a polarizing plate or the like.
The display apparatus can be driven by a line-sequential system with a driving voltage sequentially supplied with scanning. When a time (frame time) displaying a frame is constant, address time becomes shorter for each image element with an increase in the number of gate lines. Accordingly, if enhancement in resolving power of a picture is designed by increasing the number of gate lines, scanning time can not be faster than the response time of the display means such as a liquid crystal or the like. Otherwise it becomes impossible to carry out normal display. For preventing such a situation, a display process is proposed in which a capacitor is connected in parallel to each picture element, the capacitor stores a driving signal charge in a specified address period, and a driving voltage is continuously applied to each picture element beyond the specified address time. In such display structure, a gate electrode acts as a counter electrode to a drain electrode in the above-mentioned capacitor.
However, when a gate electrode is used as an opposite electrode of a capacitor, a bias voltage equal to a gate signal voltage is applied to the opposite electrode of the capacitor at the specified address time. Consequently, a writing signal is required to be set on the basis of the gate voltage. However, when a transistor assumes the state of high resistance by eliminating the gate voltage, the capacitor shows a voltage different from the drain voltage applied upon writing thereby complicating the action of the apparatus. Such complicated action is disadvantageous and inconvenient for driving a display apparatus by an exact voltage operation. Especially, controlling is difficult in the case of gradation display corresponding to applied voltage.
A display apparatus similar to the above-mentioned display apparatus is described in "IEEE Trans. on Electron Device" Vol. ED-20, No. 11 (November 1973) pp 995-1001. This article shows a structure in which a counter electrode for a storage capacitor is connected with a neighboring gate line.
In a display apparatus comprising the thus constructed display electrode substrate, a capacitor is constructed with a drain electrode and a conductive film facing the drain electrode through an insulating layer. The capacitor is connected to a gate line of a noted transistor and a neighboring gate line. Consequently, when the transistor is driven, gate voltage is alone applied to one gate line, and the other gate line is grounded. In the case of such driving process, a signal of a source line is applied to ground potential. Therefore, it becomes easy that an operating voltage of more than that of the apparatus disclosed in U.S. Pat. No. 3,824,003 is applied to a display medium. However, in this case, when a signal is applied to a gate line, the potential of a drain electrode changes depending upon the voltage condition of the gate line, so that there is the possiblity that display is affected.
In U.S. Pat. No. 3,840,695, there is disclosed a process effecting color display, in which color mosaic filters as shown in FIGS. 2(10) and 3(36) of the above-mentioned patent specification are used as the above-mentioned switching elements for drive in a display apparatus using thin film transistors.
In the conventional apparatus, parts having optical functions are assembled into a display apparatus. In such structure, there is a disadvantage that the number of steps increases since an independent step for preparing a filter is added to the conventional steps for preparing a display apparatus. When such filter is mounted on an external surface of a substrate of a display apparatus as shown in FIG. 2(10) of the above-mentioned Patent, the mosaic filter is separated from a display element portion generating an optical change between opposite substrates in an amount of the thickness of the substrate. Therefore, parallax is caused when the display is seen obliquely.